Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs.
Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating a modulated optical output. One such modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide higher data rates than other modulation formats. In a Pol Mux DQPSK modulation scheme, light having a given wavelength and a first polarization, such as a transverse electric (TE) polarization, is modulated in accordance with a DQPSK format, and combined with DQPSK modulated light having that wavelength but a second polarization, such as a transverse mode (TM) polarization. The combined light is then transmitted as an optical signal, along with other optical signals at different wavelengths, to an optical receiver node.
At the receiver node, the received optical is subject to known optical processing with components, such as an optical demultiplexer, 90 degree optical hybrid circuitry, and balanced photodetectors. In one example, the 90 degree optical hybrid circuitry may include a multi-mode interference (MMI) coupler, which has output waveguides that feed light directly to a pair of photodiodes that constitute the balanced photodetector. Efforts have been made to integrate the above-noted receiver node components onto a PIC.
Typically, the photodiodes included in the balanced photodetector include several semiconductor layers, one of which may be provided in contact with the output waveguides of the MMI coupler so that light output from the MMI coupler evanescently couples into the photodiodes. The waveguides which constitute the inputs and outputs of the MMI coupler, as well as the MMI coupler itself, may include one or more first semiconductor materials, while the photodiodes in the balanced photodetector may include one or more second semiconductor materials. Although the waveguides of the MMI coupler may not be doped, such waveguides may nevertheless have some conductivity. Thus, the MMI coupler waveguides may form an electrical path connecting the first and second photodiodes of the balanced photodetector pair, such that a current may flow between the first to the second photodiodes of the balanced photodetector pair, and the photodiodes may not properly sense incoming light.